(a) Technical Field
The present invention relates to monitoring the operational state of a fuel cell stack. More particularly, it relates to monitoring the operational state of unit cells of the fuel cell stack and the occurrence of deterioration in performance in real time and detecting nonlinearity of the current or voltage signal of the fuel cell stack.
(b) Background Art
A fuel cell is an electricity generation system that does not convert chemical energy of fuel into heat by combustion, but instead electrochemically converts the chemical energy directly into electrical energy in a fuel cell stack. The fuel cell can be applied to the electric power supply of small-sized electrical and electronic devices, for example portable devices, as well as industrial and household appliances and vehicles.
One of the most attractive fuel cells for a vehicle is a proton exchange membrane fuel cell or a polymer electrolyte membrane fuel cell (PEMFC), which has the highest power density among other fuel cells. The PEMFC has a fast start-up time and a fast reaction time for power conversion due to its low operation temperature.
The fuel cell stack included in the PEMFC comprises a membrane electrode assembly (MEA), a gas diffusion layer (GDL), a gasket, a sealing member, and a bipolar plate. The MEA includes a solid polymer electrolyte membrane through which hydrogen ions are transported. An electrode/catalyst layer, in which an electrochemical reaction takes place, is disposed on each of both sides of the polymer electrolyte membrane. The GDL functions to uniformly diffuse reactant gases and transmit generated electricity. The gasket functions to provide an appropriate airtightness to reactant gases and coolant. The sealing member functions to provide an appropriate bonding pressure. The bipolar plate functions to support the MEA and GDL, collect and transmit generated electricity, transmit reactant gases, transmit and remove reaction products, and transmit coolant to remove reaction heat, etc.
When the fuel cell stack is assembled with the unit cells, a combination of the MEA and the GDL is positioned in the center of each unit cell of the fuel cell stack. The MEA comprises an electrode/catalyst layer such as an air electrode (cathode) and a fuel electrode (anode), in which an electrochemical reaction between hydrogen and oxygen takes place, disposed on each of both sides of the polymer electrolyte membrane. Moreover, the GDL and the gasket are sequentially stacked on both sides of the MEA, where the cathode and the anode are located.
The bipolar plate including flow fields for supplying reactant gases (hydrogen as a fuel and oxygen or air as an oxidant) and transmitting coolant is located on the outside of the GDL. After a plurality of unit cells are stacked together, a current collector, an insulating plate, and an end plate for supporting the stacked unit cells are connected to the outermost ends of the fuel cell stack. The plurality of unit cells are repeatedly stacked and connected in series between the end plates, thereby completing the manufacturing the fuel cell stack.
Typically, the unit cells in a required amount are stacked together to obtain a potential required for an actual vehicle. For example, since an electrical potential generated by one unit cell is about 1.3 V, a plurality of unit cells are generally stacked in series to generate enough power required for driving a vehicle.
Meanwhile, in a fuel cell vehicle, the cell voltage is used to determine the performance, the operational state of the fuel cell stack, and the occurrence of a failure. Moreover, the cell voltage is used to control various operations such as the flow rate of reactant gas. Typically, the bipolar plate is connected to a cell voltage monitoring system through a connector or lead wire to monitor the cell voltage.
A conventional cell voltage monitoring (CVM) system directly monitors the voltage of all cells or two cells, in which the monitored information is processed by a master controller (i.e., superior controller) collecting the voltage of all cells. The CVM system monitors a voltage drop due to a failure, not the cause of the failure.
The CVM system is also used to monitor the operational state of a battery. FIG. 1 is a diagram showing a circuit configuration of a conventional CVM system, in which a total of 32 cells are connected in series.
The conventional CVM system can determine the position of a faulty cell as it directly monitors the cell voltage. However, as can be seen from FIG. 1, the conventional CVM system has a very complicated circuit configuration, and thus it is difficult to configure and maintain the system. Moreover, the system is very expensive, and it is unable to determine the cause of the failure.
Moreover, an electrochemical impedance spectroscopy (EIS) may also be used conventionally, and it is mainly used in the field of electrochemistry to determine the characteristics of electrode reactions or complexes. The EIS can obtain comprehensive information related to the properties, structures, and reactions of complexes by the analysis of the system response and is used as a useful tool in the fields of applied chemistry, medical engineering, and bioengineering.
However, the EIS is used off-line (that is, not during real-time operation of a fuel cell stack) and thus requires a long test time. Moreover, it is not suitable for real-time detection, requires a high cost, and is used only to analyze a unit cell.
U.S. Pat. No. 7,531,253 discloses a method for monitoring the operational state of a fuel cell stack in which a low-frequency current [Itest(t)] or voltage signal is applied to the fuel cell stack and the resulting current or voltage [V(t)] signal is measured to infer the operational state of individual cells of the fuel cell stack from a change in the harmonic content and the amplitude of the measured current or voltage signal.
According to this method, a drop in cell voltage is detected by a change of a linear region of the V/I (voltage/current) characteristic curve into a nonlinear state, and the possible failure of the system can be monitored by measuring all signals of the fuel cell stack.
The basic concept of this method is to monitor the operational state of the fuel cell stack on the basis of measuring only the voltage of the fuel cell stack. That is, the operational state of individual cells of the fuel cell stack is inferred from a change in the voltage of the fuel cell stack due to a change in current by frequency analysis.
As shown in FIG. 2, the voltage/current characteristics of the fuel cell stack are linear during normal operation and nonlinear under abnormal operation conditions. That is, if nonlinearity occurs in the cell voltages of the fuel cell stack, it can be determined that the operational state of the fuel cell stack is abnormal.
During operation of the fuel cell stack to which a load is connected, a sinusoidal test current [(B sin(ωt)] for frequency response is additionally applied to the fuel cell stack and, at this time, the current of the fuel cell stack is the sum of the basic operating current and the sinusoidal current [the current of the fuel cell stack=A+B sin(ωt)].
However, the above-described method uses a small sinusoidal current change as an input, and thus it has a low analytical capacity. Therefore, a method for improving the analytical capacity is required.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.